The invention relates to a throttle valve device in which a main flow passage is controlled by a main valve element in co-operation with a main valve seat, and a manually operable activation rod sealingly guided in a guide way and shiftable between an inactive position and an active position in which it contacts and displaces the main valve element off the main valve seat, thereby opening up the main flow passage.
Throttle valves of the seat type are advantageous in that they are easy to get fully tight when closed. They are also relatively compact and simple in design, which are preferable features in power tool applications. However, throttle valves of this type are typically suffering from two problems, namely: a) poor smooth start properties and, b) a relatively high opening force. Both of these problems make it difficult to start the tool smoothly, which in for instance drilling and screw tightening operations is a necessary feature to perform safe and accurate working operations.
The relatively high opening force of the valve is due to the fact that the full flow area valve has to be opened against the full air line pressure with no counter pressure from the power tool motor. One way of reducing the required opening force is to use a tilting type valve element, as described in U.S. Pat. No. 5,901,794, where the maneuver force is reduced by the lever effect of a stalk-like arm on the valve element. This type of valve, however, is not very compact in design and is suitable for pistol type power tools.
Another way of reducing the opening force of a throttle valve is to use a slide valve as described in U.S. Pat. No. 5,626,198. However, this type of valve is more expensive in manufacturing and difficult to get fully tight when in closed position. In this type of valve it is also difficult to obtain a low pressure drop and/or a large flow capacity without increasing the overall size of the valve undesirably.
The poor smooth opening properties of the seat type valve depends not only on the fact that the full area valve is opened in one step. It is also due to the fact that when starting the valve opening process a rather sudden pressure drop across the valve is accomplished which results in an abrupt reduction in the required valve opening force and an inevitably increased valve opening speed. This causes a rather quick full flow area opening.
A previous solution to this problem is described in U.S. Pat. No. 5,797,462. The described throttle valve arrangement comprises a two-step seat valve wherein the main valve element as well as a smaller auxiliary valve element are carried on a maneuver spindle, and the auxiliary valve element is arranged to control an opening in the main valve element. This type of throttle valve, however, is particularly suitable for pistol type power tools and requires a rather complicated inlet passage.